In hydrocarbon exploration and production there is a need to analyze the composite production flow from a well or a group of wells in order to investigate their origin and properties. The production system of a developed hydrocarbon reservoir includes typically pipelines which combine the flow of several sources. These sources can be for example several wells or several producing zones or reservoir layers within a single well. It is a challenge in the oilfield industry to back allocate the contributions of each source from a downstream point of measurement at which the flow is already commingled.
Other than for back allocation, compositional analysis of single sources or layers can be used to study further phenomena such as reservoir compartmentalization, invasion or clean-out of drilling fluid filtrates.
It is further known that oil samples can be analyzed to determine the approximate composition thereof and, more particularly, to obtain a pattern that reflects the composition of a sample known in the art as fingerprinting. Such geochemical fingerprinting techniques have been used for allocating commingled production from a multilayered reservoir.
There are many known methods of fingerprinting. Most of these methods are based on using a physico-chemical methods such as gas chromatography (GC), mass spectroscopy or nuclear magnetic resonance or similar methods in order to identify individual components of a complex hydrocarbon mixture and their relative mass. In some known applications, combination of gas chromatograph and mass spectroscopy (GC-MS) are used to detect spectra which are characteristic of individual components of the complex hydrocarbon mixture.
Most fingerprinting techniques as known in the art are based on the identification and quantification of a limited number of selected components which act as geomarker molecules. Such methods are described for example in U.S. Pat. No. 5,602,755A to Ashe et al. and in International Publication No. WO 2005075972. Further methods using compositional analysis for the purpose of back allocating well production are described in the U.S. Pat. No. 6,944,563 to Melbø et al.
Conventional methods of production allocation by geochemical fingerprinting techniques require the collection of clean end member fluid samples (single zone fluid samples) prior to back allocation the commingled fluids. The clean sample is collected mostly through downhole sampling using a tool with a sampling probe or a sampling tool between two packers to confine the sampling interval. Tool runs for downhole sampling are complex, expensive and may not be feasible in many scenarios, and therefore limit the general application of the known geochemical fingerprinting techniques for production allocation.
In a different branch of oilfield technology there is known a family of methods commonly referred to as production logging. Production logging is described in its various aspects in a large body of published literature and patents. The basic methods and tools used in production logging are described for example in the U.S. Pat. No. 3,905,226 to Nicolas and the U.S. Pat. No. 4,803,873 to Ehlig-Economides. Among the currently most advanced tools for production logging is the FlowScanner™ of Schlumberger.
In the light of the known methods it is seen as an object of the present invention to provide a method of determining the end member concentrations of subterranean sources or zones contributing to a total flow, back allocating and/or using geochemical fingerprinting methods without the need for prior knowledge or collection of end member fluid samples.